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powerpc/perf_event: Skip updating kernel counters if register value shrinks 

Because of speculative event roll back, it is possible for some event coutners
to decrease between reads on POWER7.  This causes a problem with the way that
counters are updated.  Delta calues are calculated in a 64 bit value and the
top 32 bits are masked.  If the register value has decreased, this leaves us
with a very large positive value added to the kernel counters.  This patch
protects against this by skipping the update if the delta would be negative.
This can lead to a lack of precision in the coutner values, but from my testing
the value is typcially fewer than 10 samples at a time.

Signed-off-by: Eric B Munson <emunson@mgebm.net>
Cc: stable@kernel.org
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
This commit is contained in:
Eric B Munson 2011-04-15 08:12:30 +00:00 committed by Benjamin Herrenschmidt
parent 09597cfe93
commit 86c74ab317
1 changed files with 30 additions and 7 deletions
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37
arch/powerpc/kernel/perf_event.c
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@ -398,6 +398,25 @@ static int check_excludes(struct perf_event **ctrs, unsigned int cflags[],
return 0;
}
static u64 check_and_compute_delta(u64 prev, u64 val)
{
u64 delta = (val - prev) & 0xfffffffful;
/*
* POWER7 can roll back counter values, if the new value is smaller
* than the previous value it will cause the delta and the counter to
* have bogus values unless we rolled a counter over. If a coutner is
* rolled back, it will be smaller, but within 256, which is the maximum
* number of events to rollback at once. If we dectect a rollback
* return 0. This can lead to a small lack of precision in the
* counters.
*/
if (prev > val && (prev - val) < 256)
delta = 0;
return delta;
}
static void power_pmu_read(struct perf_event *event)
{
s64 val, delta, prev;
@ -416,10 +435,11 @@ static void power_pmu_read(struct perf_event *event)
prev = local64_read(&event->hw.prev_count);
barrier();
val = read_pmc(event->hw.idx);
delta = check_and_compute_delta(prev, val);
if (!delta)
return;
} while (local64_cmpxchg(&event->hw.prev_count, prev, val) != prev);
/* The counters are only 32 bits wide */
delta = (val - prev) & 0xfffffffful;
local64_add(delta, &event->count);
local64_sub(delta, &event->hw.period_left);
}
@ -449,8 +469,9 @@ static void freeze_limited_counters(struct cpu_hw_events *cpuhw,
val = (event->hw.idx == 5) ? pmc5 : pmc6;
prev = local64_read(&event->hw.prev_count);
event->hw.idx = 0;
delta = (val - prev) & 0xfffffffful;
local64_add(delta, &event->count);
delta = check_and_compute_delta(prev, val);
if (delta)
local64_add(delta, &event->count);
}
}
@ -458,14 +479,16 @@ static void thaw_limited_counters(struct cpu_hw_events *cpuhw,
unsigned long pmc5, unsigned long pmc6)
{
struct perf_event *event;
u64 val;
u64 val, prev;
int i;
for (i = 0; i < cpuhw->n_limited; ++i) {
event = cpuhw->limited_counter[i];
event->hw.idx = cpuhw->limited_hwidx[i];
val = (event->hw.idx == 5) ? pmc5 : pmc6;
local64_set(&event->hw.prev_count, val);
prev = local64_read(&event->hw.prev_count);
if (check_and_compute_delta(prev, val))
local64_set(&event->hw.prev_count, val);
perf_event_update_userpage(event);
}
}
@ -1197,7 +1220,7 @@ static void record_and_restart(struct perf_event *event, unsigned long val,
/* we don't have to worry about interrupts here */
prev = local64_read(&event->hw.prev_count);
delta = (val - prev) & 0xfffffffful;
delta = check_and_compute_delta(prev, val);
local64_add(delta, &event->count);
/*